Widely used nowdays are various techniques for obtaining, normally galvanically, precious metal coatings (gold-plating and palladizing). However, galvanizing most often fails to ensure reliable coatings on parts of shaped configuration. Thus, coatings obtained by the known methods are inherently disadvantageous because of low bonding with the base metal, non-uniform thickness especially at the corners of workpieces, low hardness, and susceptibility to wear. Also, application of these known processes necessitates workpiece surface pretreatment, such as mechanical cleaning and pickling.
To improve the physical and chemical properties of coatings, new shop processes have made way to industrial use lately, one such process involving chemical and thermal treatment of metal parts by diffusive precipitation.
Diffusion coatings are surface layers characterized by low porosity and high bonding with the base metal. Diffusion coating processes can provide surface layers of various chemical compositions to guarantee such advantageous properties as high resistance to wear, sufficient hardness, tolerance to corrosive atmosphere, and high mechanical strength.
There is known a process for chemical and thermal treatment of steel workpieces accompanied by the formation of a coating thereon obtained by diffusive precipitation or deposition onto the base metal of a substance from a melt of a low-melting-point metal (cf. USSR Inventor's Certificate No. 582,329; IPC C 23 C 9/10).
Sodium is used as the low-melting-point metal of the melt, whereas platinum is employed as the substance which forms a coating in the course of diffusive precipitation.
Preferred conditions of the diffusive precipitation: temperature--630.degree. to 670.degree. C.; duration--5 to 6 hrs.
Therefore, the aforedescribed process resides in the use of a fusible or low-melting-point metal in which there are introduced other metal elements to precipitate onto the base metal and thus form a diffusion coating. An isothermal process of mass transfer takes place in which the substance is dissolved in the form of metal elements in the melt of fusible metal to be transferred and adsorbed on the surface of the workpiece being coated to bond therewith, and the element(s) are interacted with the metal of the workpiece through diffusion. As a result, a coating is formed on the base metal composed of the elements taking part in the diffusive precipitation.
However, this process fails to provide coatings of predetermined stoichiometric composition with substantially uniform coating layer thickness.
In addition, the use of the known processes of chemical and thermal treatment by diffusive precipitation with precious metals and alternative imitation metals is disadvantageous due to high losses of such metals during the processes.